RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1229 



THE VARIABILITY OF LETHAL-MUTATION RATE 



As has been indicated, even the total rate of lethal mutation per 

 chromosome, under usual conditions, is quite low, and hence has a high 

 probable error. It is therefore of interest to compare the values for the 

 A'-chromosome, obtained by different experimenters, and so to have 

 some notion of the variabihty of the process with which we are dealing, 

 under ordinary conditions. In Table 9, the control values provided by a 

 number of investigators have been put together. It is difficult with 

 such low percentages to make any satisfactory estimate of variability, 

 although certain values appear definitely low and others high. What is 

 clear is that all are of the same order of magnitude. 



Table 9. — Variation in Spontaneous Lethal-mutation Rate in the 



X-CHROMOSOME OF DrOSOPHILA MELANOGASTER 



A summary of the control data in the CI B experiments 



Authority 



MuUer (107) 



MuUer (105) 



Harris (72) 



Hanson and Heys (60) ... , 



Muller (112) 



Timofeeff-Ressovsky (178) 



Babcock and Collins (7) . . 

 Patterson (134) 



Timofeeff-Ressovsky (185) 



Oliver (125) 



Efroimson (43) 



Total 



Number of 



chromo- 

 somes tested 



3,935 



198 



986 



423 

 1,308 



984 



1,013 



479 



793 

 984 



3,362 



/986 



)453 

 '952 



1,827 

 3,058 



4,033 



371 



26,145 



Number of 

 lethals 



9 



1 



1 

 2 



2 

 4 



10 



1 



48 



Percentage 

 of lethals 



0.102 



0.202 



0.076 



0.202 

 0.690 

 0.204 



0.126 



0.261 

 0.101 



0.221 

 0.210 



0.109 

 0.130 



0.248 



0.269 



0.180 



